To produce faithful photographic reproductions of multicolored scenes, the color balance of the photographic film must be compatible with the spectral characteristics of the scene illuminant. Many photographic color emulsions are color balanced for use with natural daylight and others are color balanced for use with incandescent illumination. To properly expose a color film with an illuminant for which the film is not color balanced it is necessary to use color compensating filters.
A patent of interest for its' teaching of a method and apparatus for discriminating illuminant light is U.S. Pat. No. 4,220,412 entitled "Illuminant Discrimination Apparatus and Method" by R. A. Shroyer, et al. The method and apparatus disclosed in that patent utilizes the temporal signatures of the various light components based upon the harmonical distorted sine wave signal that is derived from the illuminant source impinging on a photodiode. The photodiode produces an electrical signal having an amplitude which varies with the instantaneous intensity of the illuminant. A means is provided for detecting the amount of harmonic distortion in the signal and for indicating the type of illumination impinging on the photodiode as a function of the distortion. In addition, the apparatus is combined with flicker ratio detecting circuitry to provide a system which is capable of discriminating between fluorescent light, incandescent light and natural daylight. The flicker ratio is the ratio of the brightest to the dimmest intensities of the light during a given time interval. Natural light, like other light emanating from a source of constant brightness, has a flicker ratio of unity. Artificial light sources, being energized by ordinary household line voltage, have a brightness which flickers at approximately 120 Hz, twice the frequency of the line voltage. Owing to the different rates at which the energy-responsive elements of incandescent and fluorescent lamps respond to applied energy, such illuminance can be readily distinguished by their respective flicker ratio.
With improvements in the state of the art it is highly desirable to have a system that is heavily digitized for performing the illumination detection. Such systems are readily manufactured incorporating integrated circuits for size and cost reductions. It is highly desirable to keep the use of analog circuits to a minimum as analog circuits are more difficult to implement. In addition, signal processing of the type used with this invention is more quickly accomplished with digital circuitry. In camera applications, the illuminant light source has to be determined almost simultaneously with the pressing of the shutter button in order to provide meaningful aperture adjustment information.
Because illumination takes the form of a periodic wave other methods of deriving signal equivalents are suggested. One patent of interest for its handling of the analysis of periodic waveforms is U.S. Pat. No. 4,301,404 entitled "Methods and Apparatus for Analyzing Periodic WaveForms" by A. J. Ley. In that patent, a periodic waveform is repetitively sampled at an integer multiple of the frequency of the waveform over a number of cycles and a sum is formed for each sample with the corresponding sample in the previous cycles. A Fourier transformation is applied to the summed samples to derive a measurement of the component corresponding to the summed samples and to the waveform in general.
Another patent of interest for its teaching is, U.S. Pat. No. 4,296,374 entitled "Wide Band Digital Spectrometer" by P. S. Henry. In that patent, the inventor utilizes a Walsh Fourier transformation to perform an analysis of a periodic waveform. The disclosed apparatus performs a wideband digital spectrometry utilizing Walsh functions to achieve a simplified method of producing the Fourier power spectrum of an input signal.
The patents referenced above talk about the frequency contents present when observing the temporal light intensity of various categories of illuminant (i.e. incandescent, fluorescent, and daylight illumination). They also show methods for detecting these frequency components and determining the illuminant category by the use of thresholds.